Honeycomb structures have a structure in which a plurality of through-holes are formed by thin partition walls, in a honeycomb shape. Of such honeycomb structures, those made of a ceramic material [e.g. silicon carbide (SiC)] are in use as an exhaust gas purification apparatus for capturing and removing the fine particle carbon present in an exhaust gas emitted from a diesel engine, etc.
Honeycomb structures have hitherto been produced by a process which comprises mixing a ceramic material with a binder composed mainly of methyl cellulose, adding, to the resulting mixed raw material, a liquid medium, ordinarily water in a given amount, kneading the mixture to obtain a plastic mixture, molding the plastic mixture into a honeycomb shape to obtain a green honeycomb structure, subjecting the green honeycomb structure to hot-air drying, and firing the dried honeycomb structure.
In this production process, however, the rate of removing the liquid medium component in the green honeycomb structure is small and the rates of drying at the surface and inside of the green honeycomb structure differ largely from each other; therefore, there has been a problem in that the green honeycomb structure comes to have a strain caused by a difference in contraction on drying between the surface and inside of the green honeycomb structure and cracks are generated in the dried honeycomb structure.
In contrast, a process is proposed in which a green honeycomb structure is dried by a drying step consisting of a combination of hot-air drying and microwave drying or dielectric drying, to remove, by the microwave drying or the dielectric drying, the most part of a liquid medium present in the green honeycomb structure. In the case of a honeycomb structure using a non-electroconductive ceramic material (e.g. cordierite), the whole honeycomb structure can be dried rapidly and uniformly; therefore, the process is advantageous in that the generation of cracks caused by drying can be prevented significantly.
In the production process, however, when an electroconductive ceramic material [e.g. silicon carbide (SiC)] is used, the applied microwave or the like is absorbed at the surface of the green honeycomb structure and drying is insufficient at the inside where the microwave or the like does not reach. Therefore, when the ratio of removal of the liquid medium in the green honeycomb structure by microwave drying or the like is high, for example, higher than 65%, a strain appears owing to a difference in contraction on drying between the surface and inside of the green honeycomb structure; as a result, there have been cases that cracks are generated in the dried honeycomb structure or significant heat generation takes place at the surface of the green honeycomb structure, resulting in complete combustion of the binder during the drying step.
On the other hand, when the ratio of removal of the liquid medium in the green honeycomb structure by microwave drying or the like is low, for example, not higher than 65%, the rate of removal of the liquid medium from the green honeycomb structure is small and a difference in drying rate appears between the surface and inside of the green honeycomb structure; as a result, a strain appears as well in the molded structure owing to a difference in contraction on drying between the surface and inside of the green honeycomb structure and it may form cracks in the dried honeycomb structure.
Meanwhile, there are proposed processes for producing a honeycomb structure, wherein a green honeycomb structure is dried by vacuum drying or freeze-drying and thereby the surface and inside of the green honeycomb structure can be dried uniformly regardless of the kind of the ceramic material used (see Japanese Patent Nos. 2612878, 3015402, etc.).
In these production processes, however, since drying is conducted uniformly, an extremely long drying time and accurate control of drying conditions are necessary; therefore, there have been problems in that quick production is difficult and a high production cost is required.